The share of uninvested cash in portfolios dropped to 5.7% from 6.1% in July, but remained "very high", BofA said.
Stocks have rallied in the past two months after a brutal first half of 2022, and BofA said August saw a big rotation into U.S stocks, technology and consumer shares, while investors sold out of defensive stocks such as utilities and consumer staples, as well as UK equities.
U.S. shares are up about 12% in the last month, but remain about 10% down year-to-date.
Recent U.S. inflation data has been better-than-expected, leading to expectations the Federal Reserve will not hike interest rates as aggressively as investors previously anticipated.